You dont have to drain anything, all your doing is removing a few lines that contain oil and coolant, just be careful not to let anything mix.
Most people find it eaiser to install the divided housings with the turbo outside the car (I sure did)
I would also HIGHLY recommend some stainless braided lines if you are removing the whole assembly. Most kits cost around $120 and they are well worth it. the OE hardlines can be a huge PITA. I have seen too many people strip out their blocks because the couldnt line up the lines perfectly.
http://enjukuracing.com/circui...4_158
I would spray some penetrating oil on the exhaust studs (after cooling) a few hours before removal also. you dont want to be drilling studs out of the head if you dont have to.
When your done make sure to bleed out all the air from the coolant aswell.